In semiconductor device fabricating processes, semiconductor wafers (hereinafter referred to simply as “wafers”) are subjected to a cleaning treatment using, for example, a chemical liquid and a rinsing liquid. The cleaning treatment typically includes, for example, a chemical liquid treatment step, a deionized water (DIW) rinsing step, and a drying step performed in this order. Prior to, or in the beginnings of, the drying step, DIW on the wafer is substituted by isopropyl alcohol (IPA). To prevent generation of watermarks, humidity in the treatment chamber is reduced before performing the drying step. To create a low-humidity environment in the treatment chamber, low-humidity high-cleanliness air called clean dry air (CDA) is supplied into the treatment chamber (see JP2007263485A).
Producing the CDA (hereinafter referred to as “dry air”) costs high. Thus, in the chemical liquid treatment step and the rinsing step in which the low-humidity environment is not required, clean air is supplied into the treatment chamber using a fan filter unit (FFU) disposed on a ceiling of the treatment chamber. Only in the drying step that requires the low-humidity environment, the dry air is supplied into the treatment chamber.
As is well known, the FFU draws air from outside the treatment chamber (air in a clean room) to form an airflow by using a fan, and filters the air by a filter provided in an airflow path to produce clean air.
If the FFU fan is stopped while the dry air is being supplied, it takes a long time for the flow of the clean air supplied from the FFU to be steadily formed in the treatment chamber, after the fan is restarted. From this viewpoint, preferably, the FFU fan is not stopped. As described in JP2007263485A, however, while the dry air is being supplied to the treatment chamber, the clean air having humidity higher than that of dry air should not be supplied into the treatment chamber. Therefore, if the FFU fan is always kept operated, a blocking damper disposed in the FFU airflow path is closed. In order to prevent increase of the internal pressure in the airflow path upstream of the blocking damper at this time, air in the airflow path is discarded from an air vent opening disposed upstream of the fan. If the filter is disposed upstream of the fan, fresh air always flows through the filter, which shortens a service life of the filter.